e eeect cr extending hot-press, urea-resin geeje with RYE EECLIR CN STRENGTH ANE) DlJRABIEITy OE THE GEUE JOINTS April 1942 row of mm im wmw OF RdM this mm is < si cr 4 series issued TO AID THE NATIONS WAR IIIIUH Nc. 11294 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin In Cooperation with the Univenity of Wisconsin EFFECT OF EXTENDING HOT-PRESS, URSA-RE3IN GLUE WITH RYE FLOOR ON STRENGTH AND DURABILITY OF THE GLUE JOINTS By DON BROUSE, Engineer INTRODUCTION Shortly after the introduotion of synthetic resins of the urea type as hot- press glues for woodworking, it was discovered that they could be mixed with fillers or extenders that did not destroy the desirable working character- istics or dry strength of the glue and that served to decrease the cost of the glue line considerably. One of the extenders commonly used at first was rye flour and its use persisted bo some extent although it was later replaced by wheat flour in many cases. Experiences in commercial practice and early tests demonstrated that joints made with highly-extended, urea-resin glues by hot pressing possessed high dry strength and appeared suitable for use in many plywood operations. The question was raised, however, as to the effect of the addition of an ex- ler, like flour, on the durability of joints that might be exposed to severe swvice conditions. Previous tests had demonstrated that joints made with pure urea-resin glues by hot pressing were highly resistant to exposures involving water at ordinary temperatures, to dampness, and to conditions favoring the development of micro-organisms. The addition of fillers of an organic type that might be affected by water and micro-organisms, however, might decrease the resistance of the resin-glue joints to these agencies. This report, therefore, covers empirical tests to determine the effect of rye flour filler in a hot- press, urea-resin glue on the dry strength, initial water resistance, durability of the joints under severe conditions of exposure. Experimental Procedure Six glAie mixtures were used to glue p ' ywood panels from which test o : mens were taken In each case, the ratio of catalyst (or hardener) to resin remained the same, but the amount of extender varied in the different ures from none to twice the amount of resin. The extender used was rye ;r and the glue was of the urea- formaldehyde type (Uformite h~$Q from the Resinous Products & Chemical Company) marketed in a liquid form in whicn the concentration of the resin was reported to be approximately 70 percent This reported concentration was no; checked but flour extender was added on the umption that 100 pounds liquid resin contained approximately 70 pounds dry resin. The mixture of resin as received, plus the amount of hardener recommended, was of a suitable consistency for spreading. When flour was added, the mixture was thickened and addition of more water was required to reduce the mixture to a suitable spreading consistency. In all the mixtures Z129^ . containing flour, the attempt was made to add such an amount of water that the consistency of the mixture was approximately the same as that of the straight glue mixture containing no flour. glues Following is a tabulation of the proportions used in preparing the Parts by Constituents weight (1) Liquid resin 100 Hardener g (2) Liquid resin 100 Hardener S Flour 7 Water 7 (3) Liquid resin 100 Hardener g Flour 17.5 Water 20 w Liquid resin 100 Hardener g Flour 35 Water 35 (5) Liquid resin 100 Hardener s Flour 70 Water 70 (6) Liquid resin 100 Hardener g Flour 140 Water 150 Percent flour in the dry glue line (based on amount of resin) 10 25 50 100 200 Of these glues, numbers 2 to 6 were prepared and used in February 1939* while number 1 was prepared and used in January 193&* Glues number 2 to 6, therefore, are all from the same shipment of resin while glue number "i was from a prior shipment of the same resin. The preparation of veneer for gluing followed the same procedure whether the gluing was done in 1938 or 1939* A quantity of l/l6 inch yellow birch veneer, selected for smoothness, firmness, and straightness of grain, was con- ditioned to approximate equilibrium with J>0 percent relative humidity at 80° F. (about 6 percent moisture content) before gluing. Eighteen 3~ply panels, 12 by 12 inches in size, were made with each glue and, after gluing, the panels were stickered and again conditioned to approximate equilibrium with J>0 per- cent relative humidity before they were cut into test specimens. Z1294 -2- The gluing conditions were held nearly constant throughout the gluing with the 6 different mixtures. The amount of glue spread varied from 19 to 2} grams per square foot with most of the surfaces receiving 20 grams per square foot (l|4 pounds per 1,000 square feet). The assembly period varied over the range from 10 to 3^ minutes, but in most cases the panels were pressed 15 minu' after they were spread. A gluing pressure of 20u pounds per square inch was used throughout. The temperature of the platens was approximately 2^0° F. and the time in the press was 5 minutes. From each panel, 3^ plywood shear test specimens of conventional design were obtained. Of these, 5 specimens were tested dry at a moisture content of approximately 6 percent, and 5 w ©re tested wet after soaking in water for ^S hours. The results of these tests were examined, all specimens from the panels containing defective joints were discarded and specimens for further tests were taken only from those panels in which the joints were well made. The test values of specimens of panels shown to be suitable for further testing were tabulated according to the glue mixture used and averages were determined for both wet and dry tests. These average values then served as controls and are shown in the first two rows of Table 1. Even though bhe specimens glued with the straight resin were prepared about a year prior to the remainder, the test values compare reasonably well with those from the specimens glued with 10 per- oent extension. The average wet test values of the two groups are well within experimental error of duplicate tests on the same glue. In dry test values, those specimens glued with the straight resin averaged 2k pounds per square inch higher in strength and 20 percent lower in wood failure than those specimens glued with 10 percent extension. Even these variations are no greater than often occur in successive tests on the same glue mixture. Since the control averages show no discrepancies, specimens glued with the straight resin may be used as a basis for comparison even though they were glued prior to the others. Tne remaining specimens of the panels selected for further tests were arranged in sets according to the glue mixture used. Holes (3/8 inch) were then bored near one end of each specimen. From each set of specimens glued with one glue mixture, k groups of SO specimens each were selected at random and each group strung on metal rods to facilitate handling through the expo- sure cycles (Fig. l). Spacers were used between specimens to provide for circulation of air and prompt change in moisture content in the cyclic tests. One such group of specimens from each glue mixture was then soaked continuously in water at room temperatures. A second group from each glue mixture was exposed continuously in a room where the temperature was main- tained at 80' F. and the relative humidity at 97 percent. A third group from each glue mixture was exposed to a repeating cycle that consisted of 2 days soaking in water at room temperatures followed by 12 days drying in an atmos- phere maintained at S0° F. and 30 percent relative humidity, and a fourth group was subjected to a repeating cycle that consisted of 2 weeks exposure at S0° F. and 97 percent relative humidity followed by 2 weeks at 80° F. and 30 percent relative humidity. At the end of each interval of 2, k, 6, 12, IS, 2k, 30, and 36 months, 5 specimens were removed from each group and tested in the plywood shear testing machine. The test values of each such set of 5 specimens Z1294 -3- were averaged and the results are recorded in the body of Table 1. Table 2 shows the same results calculated as a percentage of the dry test values for the joints made with unextended glue. The testing schedule was so arranged that, in the case of the "cyclic" tests, the testing was done at the end of the "dry half" of the cycle. The specimens that were soaked continuously were tested promptly upon removal from the water before they had time to dry out. Specimens exposed in 97 percent relative humidity were likewise tested promptly upon removal from the conditioning room before the moisture content of the samples changed appreciably. Results and Discussion This report summarizes the results up to the end of the ~$6th month. Sufficient specimens were provided to continue the test over a period of some 7 years if the joints proved sufficiently durable. Consequently the tests are being continued and further results will be available. At this period of 36 months, however, certain significant trends are apparent that can be changed but little regardless of the future course of the results. Continuous exposure to 97 percent relative humidity The effect of extending this hot-press, urea- resin glue with rye flour was most pronounced in the tests involving continuous exposure to 97 percent relative humidity and 80° F., which is a condition favoring the de- velopment of molds and wood-destroying fungi. Glue joints made with the urea-resin glue extended with 200 percent (based on the weight of the dry resin) of rye flour barely had strength to hold the plies together at the end of the second month of exposure in 97 percent relative humidity (last column of Table l). By the end of the 18th month the last specimen in the group had failed completely. This compara- tively low resistance to attack by dampness and micro-organisms is slightly lower than shown by casein glue joints in previous tests under similar conditions ._ Glue joints made with the urea resin extended with 100 percent of rye flour had lost nearly all their strength by the end of the 12th month. While several of the remaining specimens were still hanging together at the end of the 18th month, the strength was so low that, of the 5 specimens selected for test at that period, none was strong enough to give a reading on the testing machine. As compared to casein glue joints under similar conditions ,J_ joints made with this mixture of equal parts of urea resin and flour were somewhat superior but not to a marked degree. Under the con- ditions of the test, yellow birch veneer usually showed marked evidence of rot in about two years and casein glue joints usually failed before the wood had developed much evidence of decay. 1 —Exposure tests on plywood. Don Brouse. Mech. Engr. 60, S52-56, Nov. 1938. Z1294- -k- Joints made with the urea resin extended with $0 percent of rye fi showed greater resistance to attack by micro-organisms than the joints made with more extended mixtures, but at the end of 2k months the joint strength was vary low although most of the specimens were still holding together. The drop in strength of joints with 25 percent extension was gradual throughout the test period and slower than with any of the higher dilutions. By the end of the J/Oth month, however, the test values were low and the joints : no longer reliable although nearly all the specimens were holding to- re\ her. The specimens glued with a mixture containing 10 parts of flour, appeared comparatively clean and free from mold in contrast to the specimens glued with more extended mixture? which were supporting considerable mold growth on the surface (Fig. 1). At t>-e end of 36 months, the glue diluted with 10 parts of flour showed somewhat lower test values than were shown by the undiluted glue. Apparently the 10 percent dilution did not reduce to a significant degree the resistance of the glue to continuous high humidity. Specimens glued with the unextended urea resin decreased gradually in joint strength and at the end of 36 months the average test value was re- duced to some 28 percent of the original dry test values (Table 2), with zero wood failure. The resistance shown was considerably less than shown by hot-press phenolic glues in previous tests,— which, at the end of 36 months, averaged about 60 percent of the original dry joint strength with more than 90 percent of the failure in the wood. Joints made with hot-press, para- form* ldehyde-blood gluesi. averaged about 35 percent of the original dry test values, with nearly all the failure in the wood. Exposure to a repeating cycle of 2 weeks in 9 7 percent relative humidity followed by 2 weeks in "^0 percent relative humidity This exposure combines mechanical stress on the glue joint (when the plies tend to swell or shrink with moisture changes) with exposure t^ m Id attack during the time- the specimens are under conditions favoring develop- ment of micro-organisms. If th^ glue line is completely resistant to attack by micro-organisms, this test is not so severe as a soaking-drying cycle, but if the glue is not so completely resistant to mold attack, it may prove more severe than a soaking-drying cycle in which test molds and fungi have little chance to develop. In the case of joints made with hot-press, urea- resin glues highly extended with rye flour, this exposure proved more severe than the soaking-drying cycles. In many cases the specimens failed completely and slmost as rapidly as those exposed continuously to 97 percent relative humidity. Specimens whose joints were glued with a mixture containing 200 per- cent of rye flour had failed completely in this cycle at the end of the 6th month. This rate of failure is even more rapid than has been observed in previous tests_ with joints made with casein glue. Joints made with a mixture extended with 100 percent of rye flour failed completely in about 2k months. In previous tests, joints made with Z129*t -5- water resistant casein glues have held together for some 12 to -15 months under the same exposure cycle. As compared with results on other glues in previous and similar tests, the behavior of the joints with the 50 percent extension, with 25 percent exten- r and with 10 percent extension was somewhat better than that of joints made with casein glues, but not so good as joints made with hot-pressed para- formaldehyde-blood albumin glue. The behavior of the joints made with the unextended resin glue resembled that of joints made with hot-press paraformaldehyde-blood glue in previous tests in the same exposure. They did not equal the average behavior of joints made with hot-press, phenolic glues in the same exposure in previous tests. Exposure to a repeating cycle of 2 days soaking in water followed by 12 days drying in ~$0 per - cent relative humidity and S0° P . The effect of extension of the glue on the resistance of the joints -to this soaking-drying cycle was less pronounced than the effect on resistance to exposure to conditions favoring mold growth. In spite of individual irreg- ularities and inconsistencies, however, the resistance of the joints to wet- ting and drying exposure apparently decreased slightly as the amount of ex- tender was increased. As compared to previous similar tests on other adhesives, the entire group falls between the hot-pressed phenolic resins and the hot-pressed para- formaldehyde-blood glues. Joints made with mixtures containing 200 percent flour were slightly better than joints made with paraformaldehyde-blood glue and joints made with the straight urea resin were not quite so resistant as joints made with hot-pressed, phenolic-resin glue. Continuous soaking in water at ro om temperatures In general, continuous soaking in water proved to be the least severe of the exposure tests used in this experiment, but the results were reasonably consistent in indicating that the resistance of the joints to continuous soaking decreased gradually as the amount of filler added to the glue increased. The moisture resistance was high, however, even for the highest dilution with starch. At the end of 36 months of continuous soaking, joints made with the unext ended resin, with the mixtures containing 10 percent extension, 25 per- cent extension, and 100 percent extension, were all sufficiently strong to cause a high percentage of failure in the wood although the actual strength test values were only some ^>0 to 62 percent of the original dry test values of the controls. The behavior of these joints compares well with that of joints made with hot-press phenolic glues and with hot-press, paraformalde- hyde-blood albumin glue in similar exposure in previous tests. Joints made with mixtures containing 50 percent of flour and 200 percent of flour in the dry glue line were still strong enough to cause considerable failure in the wood when tested at the end of 3& rr.onths of continuous soaking. The joints made with 200 percent starch appeared considerably less resistant to continued soaking than the joints made with the less extended mixtures, but they were still more resistant than joints made with casein glues proved to be in earlier tests .£ Dry strength and initial water resistance The effect of extension with starch on initial water resistance of the joints was more regular than its effect on the resistance of the joints to continuous soaking in water. This is shown in the first two lines of entries in Tables 1 and 2. As the amount of extension was increased, the initial water resistance of the joints fell off slowly and gradually up to an extension of 100 percent. Yfith 200 percent flour, however, the decrease in water resistance was considerably greater. The effect of extension on the dry strength of the joints was the least pronounced of any of the effects investigated in this series of tests. Whi le the averages of the dry strengths showed a slow and gradual decrease a; the amount of extension increased, the differences between the averages as the ex- tension increased were small and even the mixtures containing 200 parts flour showed high strength values and high wood failures. So far as dry strength was concerned, good joints resulted even when the glue mixture was highly extended. Summary From the results of the tests up to and including ths " : 6th month of exposure the following indications were noted: 1. The resistance of the glue joints to mold attack was most markedly affected by extension with rye flour. Joints made with a mixture containing 200 parts of flour and 100 parts of resin proved scarcely equal to joints made with casein glues in resistance to mold attack. The effect of 10 percent flour was noticeable, although of minor importance, when the joints were sul jected to conditions favoring mold growth. Joints made with unextended. urea- resin glues did not prove so resistant to these conditions of exposure as joints made with hot-press ,- phenolic-resin glues. 2. The effect of extension on the resistance of the joints to wetting and drying cycles was less marked than the effect on resistance to conditions favoring mold growth but, in general, the resistance fell off gradually as the amount of extension was increased. The reduction caused by 10 percent of flour was slight and even the glue with 200 percent showed higher resistance than would be expected of a casein glue. 3- Continuous soaking in water did not prove to be a severe exposure for these urea-resin glue joints. Extensions of 3 percent and 25 percent of Z129^ -7- flour caused no decrease (as compared with unextended glue) that could be detected by joint tests during J>6 months. Joints made with mixtures con- taining 200 percent of flour were definitely lower in their resistance to continuous soaking, but still were more resistant than joints made with casein glue in previous tests under the same exposure. k-. As the amount of extension increased, the initial water resist- ance of the joints decreased somewhat more regularly than the resistance to continuous soaking in water. 5« Within the range of extensions used in this experiment, the dry strength of the glue joints decreased very slowly as the amount of extension increased. While even a 10 percent extension. caused a slight decrease, joints made with mixtures containing twice as much flour as resin were still strong enough to develop a high percentage of wood failure on test. 6. Where maximum resistance to severe exposure is essential, par- ticularly where the exposures involve conditions favorable to mold growth, the extension of hot-pressed, urea-resin glues with flour should not exceed 10 percent and, preferably, should be avoided. 7. If the service conditions require only good dry strength and a moderate degree of water resistance, the use of comparatively large amounts of flour extender is permissible. Zj 29^ -S- Table 1. — Comparative durutlllty of Joint* made with a hot-pressed urea-resln itlue used without extender and with different proportions of an extender cf ryr flour ^ — Slut ■lthout : Extended Extended Extended Extended Extended extender I with with with with with Tine \^ : 10 2> 50 100 200 of \. : percent percent percent percent peroent Exposure ^v. : rye flour rye flour rye flour rye flour rye flour Months Average teat values* None (oontrol valuee) Dryf I ! 507-6 2 E 6 12 18 24 11 6 12 18 24 3 ° 36 6 IP 18 24 30 36 2 4 6 12 16 24 3 ° 36 5 ■ «■ Ml 483-83 ■9* 458-89 45s- 90 439^88 426-71 44O-90 1 429-83 405-60 1 261- 8 489-96 1 478- Soaked continuously in water at room temperature* -* 444- 8i 441-100 401- 9o 396-IOO 340-100 316-100 298-100 3I3- 96 426- 99 i <+23- 81 I 365- 59 I 379- 84 1 328-100 1 295-100 1 277-100 1 264- 96 i 317- 66 359_ 11 350-100 350- 80 311-100 311- 92 267-100 284- 90 349-51 297-41 322-61 245-20 247-68 275-88 245-48 366- 79 35 1 *- 71 300- 62 351- 22 246- 52 240- 80 238- 82 281-100 253-12 235-13 182- 2 227- 3 204-19 137-22 172-29 176- 64 Exposed continuously to 97 peroent relative 284-16 237-20 215-19 94- t 90- 1 I 23- : 14- 1 32- humidity and SOT.- * 0-0* 25-0 13-0 0-02 0(18)' 231- 21 202- 19 149- 16 79- 0^ 0- 18- 0- 28- b Exposed to a repeating cycle that conaleted of £ days soaking in water at room temperatures followed by 12 days drying at go" F. and 30 percent relative humldltT5 1 479- 92 444- 44 446- 1 436- 12 511- 82 440- 81 362- 394- 376- 4S kit 360- 76 397- *2 3^3-39 410-23 351- 19 378- 23 366- 42 330-19 33 J-r 390- 22 345- 26 447-57 308- 4 374-42 357- M 354- 28 345-IOO 315- 2" 264-20 331- 3 289-18 292- 22 310- 1 214- 40 217-20 157- 24o- 20 166- 331- *o 2S2- 40 312-19 209-41 299- 5 178- 19 281-20 272- 20 109-20 Exposed to a repeating cycle tr.at coneUted of relative J Su nldltyS ' dative humidity followed by 2 weens in 80" 2 weefre in 60° f. and 97 percent ". e and 30 percent relative 514- 80 524- 25 476- zi 376- 21 410- 1? 248- 235- 187- 435- 60 368- 49 355- 11 250- £90- 20 309- 21 328- 60- 392- 31 1 419-30 371- toO : 321-23 329- 2 : 300- 203- 2 : 124- 184- : 108- 183- : 111- 124- : 73-0 91- : oO- 325- 26 220- 30 181- 181- 46- 13U- 0- OS. ot6)' 0(24)*: n each pair of values, the first represents average Joint strength In pounds per square Inch, the second represent* wood failure In percent. 2 "Each value la an average of 100 specimens, 5 from each of 20 panels. Specimens tested when In approximate equilibrium with 30 percent relative humidity. k -Specimens tested wet after soaking for 4g hours In water at room temperatures. h* ah value Is an average of five upeolo-na. a ~ Figures In parentheses indicate the month during which the last specimen In the group failed completely, b "In the five specimens taken for test the Joints were •till holding the piles together but the strength was •o low that the specimens could not be gripped In the testing machine without failure. Z '-' - ■<-- Table 2.- ■ Data from table 1 showing Joint strength as a percentage of the average dry the ut teet value of Joints made without lee of extenders Time of Exposure Without : Extended : Extended extender : with : with : 10 : 25 : percent : percent : rye flour: rye flour : 1 s Extended : with s 5-0 : percent : rye flour Extended with 100 percent rye flour Extended with 200 percent rye flour Months None (control values) Average teet values^ Ef I: 100- 6] Wet2 i: 96- 9t 94- 94 90- 89 89- 90 87-88 86-71 87- 80- : 85-83 t 51- 8 Soaked continuously In water at room temperature*- * 6 12 18 24 39 36 2 4 6 12 IS 24 30 36 68- 61 87-100 79- 96 78-100 67-100 62-100 59-100 62- 96 84- 99 83- 81 " II 65-IOO 58-100 55-100 52- 96 53- 66 71- 79 69-100 69- 80 61-100 61- 9a 53-100 56- 90 £8 59-41 6>6l 48-20 49-68 54- 88 48-48 Exposed continuously to 97 percent relative humidity and 80° F» 36- i it ^ 40-19 27-22 34-29 35-64 5 94- 99 77- 4S 72- 4 61- 41 6l- 31- 39- 28- 93- 83 80- 59 51- 20 5t 36 44- 20 34- 20 21- so- 43 66- 23 46- 1 42- 28- it 10- 56-16 47-20 42-19 19- IS- 1 5- 3- 6- 1 46- 21 4o- 19 30- 16 16- 0- 0. 0- 6- 0-0 5-0 3-0 °-° a 0(16)" Exposed to a repeating cycle that consisted of 2 days soaking In water at room temperatures followed by 12 days drying at 80° F. and 30 percent relative Eumldlty^ 2 4 &S 6 88- 1 12 86- 12 18 101- 82 24 87- 81 39 71- 36 7S- Exposed to a re soeat lne c. 74- 45 77- 59 91- 68 70- 41 70- 28 68-100 65- 40 59- 5 75- 76 78- 42 66-39 Si-23 77- 22 68- 26 %?l 65- 3 42- 40 57-18 43-20 56- 40 62-19 35- 19 55-20 69-19 75- 23 72- 42 If: 24 58- 22 20 20 £ 6o-i2 74-42 52-20 61- 1 33-0 41-41 33-20 ed to a repeating cycle that consisted of 2 weeks In 80° f. and 97 percent ~ atlve humidity followed by 2 weeks In 80° F. and 30 percent relative humidity^ 2 4 6 12 18 24 3 9 36 101- 80 8 103- 25 : 94- 24 : 74- 21 l 81- 12 : 49- : 46- t 37- : 86- 60 73- *9 70- 11 49- 57- 20 61- 21 65- 30 12- I 77- - 3- 3- 40- 36- 36- 24- 16- 83-30 63-23 59- 24- 21- 22- 14- 12- 64- 26 43- 30 36- 36- 20 9- 8. 0(24)- 26- 0- 0(6)* ^n each pair of values, the first represents average Joint strength expressed aa a percentage of the original dry test values of Joints made without extender, the second represents actual wood failure In percent. The wood failure values are the same as In table 1. 2 "Each value Is an average of 100 specimens, 5 from each of 20 panels. ^Specimens tested In approximate equilibrium with 30 percent relative humidity* 4 "Specimens tested wet after soaking 46 hours In water at room temperatures. *ach value 1b an average of five specimens. a "Figures In parentheses Indicate the month during which the last specimens In the group failed completely, b ~In the five specimens taken for test the Joints were still holding the piles together but the strength was so low that the specimens oould not be gripped In the testing machine without failure. 2 M 40344 F 1PB0T Figure 1 .--Arrangement of specimens for handling through the exposure cycles. The un- exposed specimens shown are, of course, from another test and are included merely to show the normal appearance of test specimens before exposure. In comparing the condition of the different groups of exposed specimens, atten- tion should be paid primarily to the condition of the individual specimens. z u 1*0771 F UNIVERSITY OF FLORIDA IIIIIIIIIIIM 3 1262 08866 6226